Feeding Preferences of Red Urchins, Strongylocentrotus franciscanus, to Drift Kelps
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Kelps and major contributors to primary production in temperate marine ecosystems. Kelps also make up a high amount of biomass and provide structure and habitat to many organisms. However, little kelp biomass is directly consumed. However, its biomass is transported via a spatial subsidy. This spatial subsidy links near-shore primary production with comparatively deep water primary consumers. One such consumer is the red sea urchin, Strongylocentrotus franciscanus. Urchins are known to feed on kelps transported by the spatial subsidy. These drift kelps may spend weeks drifting and degrading before they are encountered by an urchin. There has been little research on how the degradation or aging of kelps has on palatability to urchins. This study tests the feeding preference of red urchins to three species of kelp (Nereocystis luetkeana, Agarum fimbriatum, Saccharina subsimplex) that were both fresh and aged with the hope of gaining insight to how kelps move through the spatial subsidy. We also investigated how phlorotannin content changes as kelp age. Some research suggests that phlorotannins act as herbivory deterrent and that as kelp age they lose their phlorotannins. We found that Nereocystis luetkeana was the most preferred kelp species and that fresh kelps were preferred over aged ones. Analysis of phlorotannin content showed that there is a difference between species in phlorotannin content but the data suggests that there is little to no change through time. Further analysis indicated that urchins can detect the differences between kelps. This may mean that urchins act as a biological filter to detrital kelps as they are moved through the spatial subsidy and integrated into the food web.